BIP39 means Bitcoin Enhancement Proposal 39. The words are picked from a certain list of 2048 popular English words that may be used to the produce a binary seed (your non-public important). Standard recovery phrases use a certain list of 2048 words pursuing the BIP39 standard (Bitcoin Advancement Proposal 39). https://adsbookmark.com/story18863756/invalid-bip39-phrase-things-to-know-before-you-buy